1. Field of the Invention
The present invention relates to a zone phase correcting lens capable of concentrating laser beams different in wavelength on recording layers of optical recording mediums such as a DVD and a CD different in board thickness and to an optical head device having the zone phase correcting lens used as an objective lens.
2. Related Background Art
There are known optical recording mediums different in thickness of a transparent protective layer for protecting a recording surface or different in recording density like a CD, a DVD, and the like. A laser beam having a wavelength of approx. 655 nm is used for playing a DVD and a laser beam having the wavelength of approx. 785 nm is used for playing and recording a CD (including a CD-R). As an optical head device for recording or reproducing information to or from a plurality of types of optical recording mediums, there has been suggested an optical head device, wherein laser beams are concentrated on the recording surfaces of a DVD and a CD by using a common objective lens from the viewpoint of downsizing and lowering the cost. A CD, however, has a transparent protective layer, which is 1.2 mm thick, for protecting the recording surface. On the other hand, a DVD has a transparent protective layer 0.6 mm thick, which is thinner than the CD. Thus, the normal lens cannot suitably concentrate laser beams on the recording surfaces of the CD and the DVD. Therefore, it has been suggested to use a zone phase correcting lens having a plurality of zone refractive surfaces adjacent to each other via an adjacent step on a lens surface and formed so as to correct a phase for each of the zone refractive surfaces, as an objective lens (for example, refer to Laid-Open Japanese Patent Publication (Kokai) Nos. 2001-51192 and 2003-215447 and Japanese Patent Publication No. 3518684).
In designing this type of zone phase correcting lens, generally a lens surface whose wavefront aberration has been corrected in a laser beam having a wavelength λ1 for use in a DVD is defined as a reference lens surface. Moreover, the reference lens surface is separated into a plurality of zone refractive surfaces and these refractive surfaces are shifted from the reference lens surface in the optical axis direction. In this regard, as shown in FIG. 4, the height measurement d of an adjacent step formed between the zone refractive surfaces adjacent to each other is defined so as to satisfy the conditions: the product of a vertex step height measurement δ and (N1−1) is an integral multiple of the wavelength λ1, where N1 is the lens refractive index in the laser beam having the wavelength λ1, and the aberration of the CD is at a minimum, supposing that the vertex step height measurement δ is a distance between the vertex obtained by extending the zone refractive surface toward the optical axis and the vertex of the innermost surface. Therefore, stepped discontinuous wavefront aberration occurs in a laser beam having a wavelength λ2 for use in a CD without deteriorating the wavefront aberration in the laser beam having the wavelength λ1 for use in a DVD. Thus, in order to reduce the aberration, the phases of the zone refractive surfaces are corrected so that the aberration is allotted appropriately to the aberration on the DVD side. Thereby, it becomes possible to control the shape of aberration of the laser beam having the wavelength λ2.
Conventionally, the step height measurement δ is generally set so as to satisfy the following formulas:|m×λ1−δ×(N1−1)|<0.15×λ1 |n×λ2−δ×(N2−1)|<0.15×λ1 
where
λ1=Wavelength of first laser beam for DVD
λ2=Wavelength of second laser beam for CD
N1=Refractive index of lens material in first laser beam
N2=Refractive index of lens material in second laser beam
m, n=Integer of 0 to 3, and
δ=Height measurement of vertex step
If, however, the conventional configuration is represented using the height measurement d of an adjacent step formed between zone refractive surfaces adjacent to each other, instead of the height measurement δ of the vertex step, the adjacent step height measurement d is low as represented by the following formula:|d×(N−1)|<1.3×λ
where
λ=Wavelength of laser beam, and
N=Refractive index of lens material in laser beam having wavelength λ
Thus, the conventional lens is insensitive to wavelength fluctuations. Therefore, it has a problem that, if the wavelength of a laser beam emitted from a semiconductor laser varies due to a temperature change, it is impossible to offset the third-order spherical aberration and the fifth-order spherical aberration effectively.